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Tuesday, May 29, 2012

2. What’s the point?
We utilize the x-rays produced by the electron microprobe for many research applications.
There are other techniques, similar in some ways, that are worth discussing, that utilize x-rays for secondary x-ray fluorescence. Two in particular are:
XRF (X-Ray Fluorescence), where x-rays from a sealed tube are used to produce x-rays by secondary fluorescence in samples of interest (traditionally a macro-technique)
Synchrotron Radiation, where electrons are accelerated in ~10s-100s meters diameter rings, and then made to produce highly focused beams of extremely intense x-rays or light, which are then fed into many different types of experiments.
The benefits of secondary x-ray fluorescence include very low detection limits (10s of ppm easy in 10 seconds, no backgrounds)

3. XRF Basics
The basics of XRF are very similar to those of EPMA—we are dealing with characteristic x-rays and continuum x-rays— with the exception that we are doing secondary fluorescence : x-ray spectroscopy of our samples using x-rays coming out of a sealed tube to excite the atoms in our specimen.
The big difference is that
there is NO continuum generated in the sample (x-rays can’t generate the Bremsstrahlung), and
we are using BOTH characteristic x-rays of the sealed tube target (e.g., Cr, Cu, Mo, Rh) AND continuum x-rays to generate the characteristic x-rays of the atoms in the sample.
XRF has been a bulk analytical tool (grind up 50-100 grams of your rock or sample to analyze), though recently people are developing “micro XRF” to focus the beam on a ~100 mm spot.

4. X-ray Sources
The standard X-ray tube (top right) was developed by Coolidge (at GE) around 1912.
It is desirable to produce the maximum intensity of x-rays; a Cu target tube might be able to deliver 2 kW. The limiting factor is the heat that the target (anode) can handle; cold water is used to remove heat.
Higher power can be delivered by dissipating the heat over a larger volume, with a rotating anode (bottom right). However, this is not normally used for XRF.

5. X-ray Attentuation
This figure shows the attenuation of the X-rays in the target (sample).
In addition to photoelectric absorption (producing characteristic X-rays and photoelectrons [=Auger electrons]), the original X-rays may be scattered.
There are two kinds of scattering: coherent (Rayleigh) and incoherent (Compton).

6. X-ray Scattering
Coherent scattering happens when the X-ray collides with an atom and deviates without a loss in energy. An electron in an alternating electromagnetic field (e.g. X-ray photon), will oscillate at the same frequency (in all directions). This is useful for understanding X-ray diffraction (in depth).
Incoherent scattering is where the incident X-ray loses some of its energy to the scattering electron. As total momentum is preserved, the wavelength of the scattered photon increases by the equation
(in Å)
where f is the scatter angle. Since f is near 90°, there will be an addition peak from the main tube characteristic peak at about 0.024Å higher wavelength